1. Field of the Invention
This invention relates to a circuit arrangement for igniting and operating a discharge lamp, provided with
input terminals for connecting a supply source, PA1 commutator means provided with output terminals for connecting a lamp PA1 to be operated,
which commutator means comprises at least two (2) switching means which are alternately in a conducting and a non-conducting state, each of the two switching means being connected to a connection point which is connected to one of the output terminals.
2. Description of the Related Art
A circuit arrangement of the kind mentioned in the opening paragraph is known from EP-A-0398432=U.S. Pat. No. 5,087,859. The known circuit arrangement is particularly suitable for operating a high-pressure discharge lamp of a projection TV installation (PTV). An important property of a high-pressure discharge lamp is that acoustic resonances will occur under certain conditions. Such resonances give rise to instabilities in the discharge arc of the lamp, and thus in the light output. In extreme cases, said resonances will lead to instabilities of such an intensity that the lamp extinguishes. The resonances may also give rise to attacks on the discharge vessel of the lamp such that the end of lamp life is reached prematurely. The risk of acoustic resonances occurring is avoided in the known circuit arrangement in that the commutator means are operated at a fixed frequency, which frequency lies within a resonance-free band of the lamp.
The commutator means in the known circuit arrangement is formed by a bridge circuit with four switching means which are interconnected in pairs via respective connection points and which are switched alternately into a conducting and a non-conducting state. The bridge circuit comprises a bridge branch between connection points in which the output terminals for connecting a lamp to be operated are incorporated. To control the power consumed by the lamp, the known circuit arrangement comprises a controlled power stage which supplies a DC voltage to the commutator means. A disadvantage of this is that an additional power stage is required. An alternative mode of power control is a control of the switching frequency of each of the switching means of the commutator means. This is practically impossible in the operation of high-pressure discharge lamps on account of the problem of acoustic resonances as described above. In practice, bridge circuits with two switching means are widely used for operating low-pressure mercury discharge lamps. It is proposed to control the power consumed by the lamp, and thus the luminous flux of the lamp, through a control of the switching frequency of the switching means. It is found to be necessary to increase the switching frequency very strongly compared with that obtaining in nominal operation of the low-pressure mercury discharge lamp if the luminous flux is to be controlled over a wide range in this manner. An important disadvantage of this is that the use of such high switching frequencies leads not only to a quick variation in light intensity (flickering) which is unpleasant to the human eye, but also to a strongly increased intensity of the radio frequency interference (RFI). Although it is possible to counteract the occurrence of radio frequency interference by means of additional, specific interference suppression circuits in addition to usual filter circuit means, the use of such interference suppression circuits leads on the one hand to a considerable power loss and on the other hand to a disadvantageous rise in the cost price owing to a strongly increased complexity of the circuit arrangement.
It is an object of the invention to provide a circuit arrangement of the kind described in the opening paragraph with a control of the power consumed by the lamp in which the above disadvantages are avoided.